1. may be due to not enough sources or ISP p2p shaping. If the downloads are missing, check your settings. Make sure incomplete files are set to be kept for more than the default 7 days. I set it at 999 days.
To find your downloads, go to your LW Library window, select Saved Files folder & press the Explore button. Sample image here: Using Library Explore button to access downld folder (click on link) & the folder will open up for you. You will probably find them there.

2. What program are you using for previewing? I'd suggest you go to Library window, select incomplete folder, find the incomplete file & select it. Then press Launch. This has the advantage of not producing a preview file.

For more help, we need answers to both the questions I asked above & also these ones: System details - help us to help you (click on link)